Filipe S. Dias, Patricia María Rodríguez-González, Luís Borda de Água

Conservation science usually devotes little attention to common species, which are crucial in determining ecosystem structure and function. Dominant species are a particular type of common species that affect ecosystem functions proportional to their abundance or cover. In this study, we examined how human disturbance affects the cover of the dominant riparian species in 404 sites located in multiple streams across continental Portugal. Specifically, we asked if the percentage of cover of the dominant species is higher or lower in the least disturbed areas. We performed the analysis using Extreme Value Theory. We found that the percentage of cover of the dominant species was significantly higher in the least disturbed communities compared to the disturbed communities. Riparian vegetation clearing is common in more humanized and disturbed areas which may explain the reduction in the cover of the dominant species. In the least disturbed areas, vegetation clearing is less frequent, allowing more growing time and permitting the dominant species to have a higher cover percentage. Since we found significant differences in the percentages of cover, we examined the identity and ecological characteristics of the dominant species. We found that the common alder was the species that more often had the highest percentage of cover regardless of the disturbance level. However, we found substantial differences in the identity of the remaining dominant species, which we believe were linked to their ecological requirements.